Method of producing age-hardened magnesium-base alloy



Dec. 16, 1952 A. H. HEssE METHOD OF PRODUCING AGE-HARDENED MAGNESIUM-BASE ALLO-Y Filed may 1o. 195o 2 SHEETS-SHEET l ATTORNEY A. H. HESSE Dec. 16, 1952 MET-Hon oF PRODUCING AGE-"HARDENED MAGNESIUM-BASE ALLOY Filed May 1o, 195o 2 SHEETS- SHEET 2 .u oom,

Il) Ui MM2/F2221.

pm m PM n N n A.

Patented Dec. 16,1952

UNITED STATES PAT l-NT OFFICE g 12,522,049 METHOD oF PRoDUoING AGE-HARDENED i MAGNEsIUM-asr: anno YEv .Hi lHesse,Y La- Grangeg" Ill.,assignor, by`v mesne assignments, to Mathieson Chemical Corporation, New York Virginia N. Y., a corporation of Application May 1o, 1950;*seria1No. 161,247

s r u 21C "L ilhijsinvention relates to the age-hardening of magnesium-base alloys compris-ing magnesium and lithium' in theratio tooneanotherA of l5 :.1 .te 19:1. The invention is based upontlie discovery of magnesium-bas'ealloy compositions.consisting 5 of lithium and orie or more other metals resulting in the formation Iof; anevverystal phase which I hereina'fterfcall thet a,f.lphase`, and the The'additiva@"Massa giuaay 'refother metals o f the .group consisting,

' #lithium binary alloy" produces, the

Iig tothe invention invention prei- 4 h mtgnsiufbase alleys -..F1aieg this th: ph'a'seresulting in a precipitation lof at least an part 5T shows'the sharp, wiiqesnee pattern ofthe theta phase typically observed mover-aged alloys. Y Pattern 3v was obtained Vfrom a cadmium ternary a1lo'y, -whereas patterns and-5 were ob-g tained from'a silver ternary-alloy. Lg;-

In Fig.V 1, the diffraction lines making up 'th-e various patterns are plotted with the visually estimated relative intensity shown on the verticallaxis and the -interplanar spacingi shown in angstrom units onthe--hor-izontal axis (an Angstrom, abbreviated is equal to 10-3 centi meterln magnesium-base alloys which con- .z tain theftheta phase, the alpha phase and beta p'hase are(also normally presentto'some extent h depending.' uponithe Composition. olf; the l dmiumi; suya.. and hammam t6 the alloys'. AMagnesiumfbase a1loys.co m 2D etheta phase'. and,areagehardenable 25 oi the theta: phase4 in the age-hardened 30 hasbserridenues waant 40 type,this lattice constant for theta phaselx'nay be 45 nightly greater" than", 6.92 er' somewhat gessi" thence A: Astindicatedirigl; pattern is a'iypcal'A pattern`of 'th beta cph`as'e" r;- hqdybelf teredcubi'c lattice structur, rpattern 2 i'stypical ofthe hexagonal close-packed magnesium 50 'tureV known: s 'alpha pirasefpaa emplary'of a'diiius'pattern ci," HetaI p s e structure erf-maximum iattic'n'stant;pattrrrt illustrates a diffuse theta phase pattern having a relatively smaller lattice constant, and pattern 55 andthe lili-'ray diffraction photgram of the sample examinedwll be a mixture of the X-ray diffraction patterns of all the phases present however, the patterns of these phases may be all separated :from one another by usingga; device.

such as the graph in Fig-fl. t A

vlatterrrfiand 4 arediiisetheta-phase pat- 1 terns, as represented bythe` broad bands indie, i cated for each diffraction ma;,;i mum,.vvhich a1 e ohtainedlupon the examination-of alloys which have been Lage-hardened, Ijhe diTractiOnlines in pattern 4V are each siighuyto ui'right of the corresponding lines shown in pattern 3, since the lattice constant-of thev alloy which produced patey tern\4v is-somevvhat `lessl thanY that in the alloy whichproduced pattern A still further shift of the diractionlines to the right -isV shovvn in pattern syvl-iih -is-a pattern fior thesamealloy as that of -pattern- 4, but after overagingfat a higher temperatures The sharpness of patternv 5 is typical ofthetaphase patternsobtained in examination of over-agedalloys, and it is because of the sharp' diffraction patterns so obtained that it is usually preferable to examine the alloys in theroveriaged conditionto determine the pres-- ence yof vtheta phase. j One of the outstandingcharacteristics that identies the theta. `phase and differentiates it from other face-c entered-cubi structures isthe.. high intensityoi the third diiractiqn line from the left shown in patterns 3, 4,' and'i'I (recognized by persons familiar With'the art as having Miller indices 2220)',Y asY compared With the intensity lof the first diffraction line from the left (Miller indices 1 11 in patterns 3, tY and 5 yin the usual pattern o f a metal or terminal solid solution. hav,-v

ing a f acecentered-cubic structure, the third line (Miller indices 220) has on ly aboutone- Sv third the intensity of the irst line. For theta phase, however, the intensity of these two lines `is almost equal. Furthermore, a face-centeredcubic metal or terminal solid solution would have a much smaller lattice constant. A further distinctive feature of the theta-phase diffraction pattern is the presence of a line at Miller indices of 200, shown as the second line from the left in patterns 3, 6 and 5. The simultaneous existence of these two features helps to readily identify the theta phase. As will be apparent to those skilled in the art, the diffraction patterns represented in Fig. 1 are merely typical, and slight departures therefrom in intensities and lattice spacings will be found in various types of theta-phase patterns.

In the following Table l, the Miller indices, the visually estimated intensities, the same relative intensities placed on a numerical scale, and the interplanar spacings of the lines found in a typical theta-phase pattern are tabulated. The X-ray tube had an iron target or anode and the sample, photographed in a Debye camera, comprised filings filed under 20G-proof ethyl alcohol and dried. The alloy employed contained 4% zinc, 6% cadmium, 6% silver, the balance being magnesium and lithium in the weight ratio to one another of 7:1. Patterns of theta phase can also be observed in diffraction patterns obtained on an X-ray diffraction spectrometer, either from block samples or from filings.

TABLE 1 Table of X-ray diffraction Zines observed for theta phase Interplanar spacing in augstrom units when lattice constant equals 6.75

Relative intensity of line on numerical scale Visually estimated intensity of line Very, very faint Faint Faint to medium-faint Very, very faint Medium-faint Very faint Faint The points of distinction for identifying the theta phase brought out above in connection with the patterns in Fig. 1 are further emphasized by the above table. The line having Miller in dices of 111 had a relative intensity of 1.0, whereas the line having Miller indices of 220 had a relative intensity of 0.3. The line having Miller indices of 200 had an intensity of 0.3.

I have found that none of the magnesium-lithium binary alloys form the theta phase, but that a large number of magnesium-base alloys containing other added metals do form theta phase and respond to age-hardening treatment, as shown in Table 3.

In carrying out this process, the usual pretreatments such as hot working, cold working, extruding, etc. may be employed, but are not requisite to the age-hardening treatment. In general, it is desirable to solution heat treat the alloy, that is, to put all or a major part of the theta phase into solution. The alloy may be hot worked or subjected to any other treatment which aids in dissolving the theta phase. The solution of theta phase involves two inter-related factors, namely, time and temperature. Below certain temperatures, theta phase will not go into Solution regardless of time and this solution temperature varies from one alloy to another. As the temperature is increased, the time required for complete solution is decreased. Usually, temperatures of from 400 to 600 F. are satisfactory and holding periods of from a few minutes to 16 hours may be employed. The following Table 2 shows the effeet of the solution treating temperature on an alloy containing 66.9% magnesium, 11.1% lithium, 16% cadmium, and 6% silver.

After all of the theta phase has gone into solu' tion, the alloy is quenched with suflicient rapidity to prevent precipitation of the theta phase. This is normally accomplished by a water quench, although any other quenching means which eifects a sufficiently rapid quench can be employed.

The above table shows that below 400 F. theta phase was not completely dissolved, but solution treatment at 400 F. produced an alloy containing a finer grain size than the same alloy solution treated at 500 F. Grain size appears as a direct function of the solution treating temperature.

The solution heat treated alloy must then be reheated at a temperature substantially below the solution temperature in order to bring about the precipitation of the theta phase. For most of these magnesium-base alloys, a temperature of about 150 F. produced the best age-hardening conditions, although certain alloys age-hardened as high as 300 F. and others age-hardened at 70 F. As is shown in Table 3, however, alloys which hardened only at '70 F. required such a long period of aging that the process would be commercially impracticable. On the other hand, alloys which age-hardened at 300 F. were generally quite unstable. Aging times of about hours have been found preferable. Referring nowv to Table 3, it is evident that for both heat Nos. 25 and 32 higher aging temperatures hardened the alloys more rapidly than the lower aging temperatures. In both cases, however, aging Aat F. produced as high or higher hardnesses than aging at and 200 F., so that the ultimate hardness obtained was not affected materially by the aging temperature. This is probably du'e to the fact that the maximum hardness is attained before all of the theta phase precipitates, and the precipitation of additional theta phase merely over-ages the alloy.

In order to enable those skilled in the art to better practice the present invention, the following detailed examples are set forth:

EXAMPLE l An ingot of heat No. 25 with a composition of 66.9% Mg, 11.1% Li (6 Mg/Li), 16% Cd, and 6% Ag was extruded at 450 F. from a diameter of 2 inches to a strip of 1A inch thick by M; inch width. The strip was given one hotrolling pass at 30% reduction at 550 F., air cooled to room temperature and then reduced by cold rolling 15%. The cold-rolled strip was solution treated by holding for 16 hours at 500 `5 F. To protect it from oxidation during solution treating the strip was coated with a silicone varnish. After solution' treatingthe stripl was water quenched andvagled, ai'r at temperatnres of 7.06.11', 150. and .390 F. As ,Shown 111 Tabl, 3; it required 2000 1 1' at TQW for 'this' 85 hours were required to reach the same maximum hardness yat 150 F. .At `300, F. this alloy agedrapidly 15 hurs) but "oly increased from 6 iir'tslies square to a cross-section or 4 inches by 1% inches. The forged ingot was machined to clean up at n1.462 inches thickness land-then hot jolledat 550 ,F. to.. 0.668 inch thickne`ssin-,the crossior'ging direction. It was then turned and hot rolled at.550 F. in thefforging direction to 0.250 inch thick. The temperature was `then dropped to 400,F. and rolling continued in the forging directionto a thickness of 0.055 (inch. Tensile specimens from this sheet -were' coated with va silicone varnish and solutiontreated by holding at 400 LF. for 16 hours.` The solutiontre'ated hardness was 73 Rockwell. E. ,On aging atiw150 F. for 135 hours the hardness was increased to 93.5 Rockwell E. In the aged state, the tensile yieldstengoth Was 472800 ic. s. i., the ultimate tensile 'strength was 49,200 p. s. i.; and elongationy was 1.8%. 2

'I'his application is a continuation. n-part of my c'pending'appucauo sfii N6. 792232," d December 17, 1947 (now abandoned).

(See footnotes at end of table.)

TABLE `s-c mtmued Intended composition l Time to percent 553g Cold Solution Aging ggg' Maxireach Tllnft Heat Mg] t mfg rolling treating temas mum maximum Duration Aged hardness at No. Li in* reductempera pera- Sohb aged mum aged of test, end of test 1eme tion, ture, ture, tion hardaged hardness hours Cd Ag Other s percent F. F. trated ness harrldness, hours ours 300 60 60 gid not age harden.

70 67 88 2 000 500 2, 00 24".. 6 12 6 550 15 600 150 l 8g 22g 300 7o 75 92 2, ooo 50o 21 50o 92. 25- 6 16 6 550 15 G00 i 150 85 50g 300 1.

300 70 70 Did not age harden.

70 64 85 2 000 500 2, 500 85. 27 6 4 6 4InNa 550 8.5 600 150 g3 I 2 Egg 2, 300 5 1 7o 76 93 2, 50o (l) 2j 50o 93. 28--.. 6 l2 2 lAl 550 15 600 150 75 12 135 5 300 75 70 so 95 1, 50o 1,000 2; 50o 95. 29 6 16 2 1A] 550 15 660 150 39 93 15 130 5 300 9 88 l.

300 60 ....566 5566. gid not age harden.

300 67 67 gid not age harden.

70 76 93.5 2 500 1 2 5 .5. 32..-. 6 12 8 0.5A1 550 7. 5 600 :1x50 75 95 I 100 90(2) 00 75 91 70 77 96. 5 2, ooo 50o 2; 50o 96.5. 373.--. 6 6 6 4Zn 600 15 600 150 g5 96 3g l 300 5 77 70 68 92 l, 500 1, 000 2, 500 92. 34..-. 6 4 4Z114Ca 550 15 4 500 150 6g 72 8g 155 300 6 69 70 66 91 900 2, 600 3, 500 91. 35--.. 6 4 4Z!! 550 15 500 150 65 70 82 180 l, 000 66. 300 65 67 1 2. 5 1, 000 58.

l The hardness of this alloy Will probably remain at this level indefinitely.

I A second phase which could not be put into solution at 500 F. could be dissolved at 600 F. A second phase could not be put in solution at either 500 F. or 600 F.

NoTE.-All hardnesses in this table are reported on the Rockwell E scale.

I claim:

1. The method of producing an age-hardened magnesium-base alloy consisting of magnesium and lithium in the ratio to one another of 5:1 to 10:1 and at least one metal of the group consisting of 4-10% zinc, 424% cadmium, 0-12% silver and 412% aluminum and having a precipitated theta phase which comprises heating the alloy to dissolve at least a major portion of the theta phase, quenching the alloy to prevent f precipitation of the theta phase, and thereafter heating the quenched alloy to a temperature above F. but below 300 F. to precipitate at least a part of the theta phase to age-harden the alloy, said theta phase being identied by X-ray diffraction pattern as a face-centered cubic structure having a lattice constant ranging from 6.5 units to 6.92 units.

2. The method of producing an age-hardened magnesium-base alloy consisting of magnesium and lithium in the ratio to one another of 5:1 to 10:1 and at least one metal of the group consisting of 4-10% zinc, 4-24% cadmium, 0-12% silver and 4-12% aluminum and having a precipitated theta phase which comprises heating the alloy to a temperature of from 400 F. to

ALFRED H. HESSE.

REFERENCES CITED The following references are of record in the le of this patent:

FOREIGN PATENTS Country Date Australia Jan. 15, 1942 OTHER REFERENCES Hume-Rothery et al., Journal of the Institute of Metals, published by the Institute of Metals, London, England (1945), volume 71, pages 589-601.

Number 

1. THE METHOD OF PRODUCING AN AGE-HARDENED MAGNESIUM-BASE ALLOY CONSISTING OF MAGNESIUM AND LITHIUM IN THE RATIO TO ONE ANOTHER OF 5:1 TO 10:1 AND AT LEAST ONE METAL OF THE GROUP CONSISTNG OF 4-10% ZINC, 4-24% CADMIUM, 0-12% SILVER AND 4-12% ALIMINUM AND HAVING A PRECIPITATED THETA PHASE WHICH COMPRISES HEATING THE ALLOY TO DISSOLVE AT LEAST A MAJOR PORTION OF THE THETA PHASE, QUENCHING THE ALLOY TO PREVENT PRECIPITATION OF THE THETA PHASE, A ND THEREAFTER HEATING THE QUENCHED ALLOY TO A TEMPERATURE ABOVE 150* F. BUT BELOW 300* F. TO PRECIPITATE AT LEAST A PART OF THE THETA PHASE T AGE-HARDEN THE ALLOY, SAID THETA PHASE BEING IDENTIFIED BY X-RAY DIFFRACTION PATTERN AS A FACE-CENTERED CUBIC STRUCTURE HAVING A LATTICE CONSTANT RANGING FROM 6.5 A. UNITS TO 6.92 A. UNITS. 